At present, radio communication systems such as a cell-phone system and a radio MAN (Metropolitan Area Network) are widely used. For attaining a further speeding up and large capacity of radio communication, lively discussion is continuously performed about a next generation radio communication technology. For example, the 3GPP (3rd Generation Partnership Project), which is one of international standardization organizations, proposes a standard referred to as LTE (Long Term Evolution), and a standard referred to as LTE-A (Long Term Evolution-Advanced) that is an evolution of LTE (see, for example, 3rd Generation Partnership Project, “Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA)”, 3GPP TR 36.913 V8.0.1, 2009-03).
In a radio communication system including a base station and a mobile station, a relay station which relays radio communication may be provided between the base station and the mobile station. By providing a relay station, an area (dead spot) in which radio communication is difficult due to radio propagation blocking caused by buildings is covered, a range of a cell covered by a base station is expanded, and communication throughput is improved.
However, in a relay station, interference (which may be referred to as self-interference) may occur between a transmission signal of its own station and a reception signal. Suppose, for example, that a frequency band used between a base station and a relay station and a frequency band used between the relay station and a mobile station are overlapped with each other. In this case, a radio signal transmitted to the mobile station comes into a receiver of the relay station, and as a result a radio signal may not correctly be received from the base station. To cope with the problem, the relay station is proposed to be controlled in such a manner that reception of a radio signal from the base station and transmission of a radio signal to the mobile station are not performed at the same time (see, for example, section 9. 3 of 3rd Generation Partnership Project, “Feasibility study for Further advancements for E-UTRA”, 3GPP TR 36.912 V9.0.0, 2009-09).
Another radio communication system including a base station and a mobile station may provide a configuration in which a procedure of random access from the mobile station to the base station is specified. In the random access, the mobile station accesses the base station without being dedicatedly allocated a radio resource by the base station (see, for example, section 10. 1. 5 of 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description”, 3GPP TS 36.300 V9.3.0, 2010-03).
As a random access preamble (which may be referred to as Msg 1), for example, the mobile station transmits to the base station a signal sequence selected from among a plurality of candidates through a predetermined random access channel. The base station, having received the Msg 1, transmits as a response a random access response (which may be referred to as Msg 2). Note that at this time, the base station does not recognize the transmission source device of the Msg 1. The mobile station, having received the Msg 2, transmits to the base station a message (which may be referred to as Msg 3) including the identifier of its own station. The base station, having received the Msg 3, transmits to the mobile station a message (which may be referred to as Msg 4) as a response.
Here, an interval until the base station sends back the Msg 2 from reception of the Msg 1 and an interval until the base station sends back the Msg 4 from reception of the Msg 3 are not fixed, and preferably stay within a predetermined allowable range. Based on this flexibility, the base station may perform scheduling and efficiently transmit the Msg 2 and Msg 4. While the base station may transmit the Msg 2 or Msg 4, the mobile station monitors a radio signal from the base station and detects the Msg 2 or Msg 4 (see, for example, section 5. 1 of 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”, 3GPP TS 36.321 V9.3.0, 2010-06).
Think of a radio communication system which includes a base station, a relay station, and a mobile station, and in which the relay station performs random access to the base station. At this time, there arises a problem that how the base station and the relay station preferably perform a random access procedure. Specifically, in the conventional random access, timing at which a base station may send back a message varies. If the message is sent back at timing at which the relay station transmits a radio signal to the mobile station, the relay station may not correctly receive the message due to self-interference.